Construction costs justified / comparable?

2) Should one expect similar costs if building the same number of cubic meters, only distributed over the basement, ground floor, and upper floor?

No, basement sqm/cbm usually cost more than sqm above ground level.

Room height is also generally cheaper to achieve than more cubic meters gained through a larger footprint of the building.

3) Are exterior facilities generously or rather tightly calculated with a flat rate of 35K?

It clearly depends on the plot size and things like terrace size, driveway, etc.

Nothing else can be said so generally.

For 240 square the price is more than valid, I dare say.